1. Field of the Invention
The present invention concerns a method for determination and evaluation of a shim parameter set for controlling a shim device in a magnetic resonance apparatus.
2. Description of the Prior Art
For image acquisition in magnetic resonance apparatuses, the homogeneity of the magnetic field in the region of interest is of great importance. Small deviations in the homogeneity can lead to large deviations of the frequency distribution, such that qualitatively substandard images are acquired.
Shim devices are known in order to improve the homogeneity in the volume of interest for an image acquisition. When a magnetic resonance apparatus is installed at its installation site, fields and field sources as well as materials present in the environment can limit the built-in homogeneity of the magnetic field of the apparatus, in particular around the isocenter. Upon installation and start-up of a magnetic resonance apparatus, the shim device is set (frequently in connection with measurements) such that as optimal a homogeneity as possible is established.
However, the subject from which image data are to be acquired itself represents a further inhomogeneity source. When, for example, a person to be examined is introduced into the magnetic resonance apparatus, the matter of the body disrupts the homogeneity again. In order to counter this problem, it is known to use re-adjustable shim devices. In particular, electrical shim coils are known that, controlled with various shim currents, generate various compensation magnetic fields in order to improve the homogeneity.
In order to shim the field in a manner specific to the subject, in order to control the shim device by means of a first shim parameter set acquired during the installation and start-up of the magnetic resonance apparatus, it is typical to first conduct a measurement of the field distribution using the magnetic resonance apparatus itself after the person to be examined has been introduced into the patient admission of the magnetic resonance apparatus. Starting from the first shim parameter set, an optimized shim parameter set is determined by a control device dependent on the measured field distribution. Using this optimized shim parameter set, the shim device is then controlled in order to optimally achieve an improved homogeneity. Various algorithms are known to determine this second shim parameter set that is optimized in an ideal manner, but certain problems arise in the known methods.
Under some circumstances the employed algorithm does not converge at an optimized shim setting. For example, this occurs when the first shim parameter set has values too far removed from the optimal possible values; a particularly large inhomogeneity thus exists. A further error source is in the measurement of the field distribution, since measurement errors can also easily lead the algorithm to false results.
In the prior art it is known to effect one or more further iterations, meaning to possibly effect further field measurements and to newly apply the algorithm until a user is of the opinion that the last determined shim parameter set is optimal.
However, with the methods known today it cannot be established whether the “optimized” second shim parameter sets are even actually optimal or whether they might even represent a worsening relative to the starting shim parameter set (in particular the first parameter set). This only after an image is generated and the subjective image quality is determined. Although the algorithm should have determined the theoretical minimum for the inhomogeneities of the field, in practice measurement uncertainties and fluctuating signal quality can misdirect the algorithm and a worsening of the shim situation can occur. A user is, so to speak, “blind” with regard to these cases.